High gain rare earth doped phosphate glass optical amplification fibers

ABSTRACT

In accordance with the present invention, there is an optical amplifier fiber comprising a core manufactured from a phosphate glass doped with a rare earth element and a cladding manufactured from a phosphate glass surrounding the core. The core has a radiative lifetime in the range of 7 to 9 milliseconds at 1535 nm, a fluorescence lifetime of greater than 7.5 milliseconds at 1535 nm. The optical amplifier fiber has a diameter ratio in the range of 0.036 to 0.044, a transformation point difference of the core and the cladding, measured in (°C.), less than 5%. Further, the optical amplifier fiber has a coefficient of thermal expansion, measured in (/°C.), difference between the core and the cladding is less than 2% and an absorption cross section in the range of 0.60×10 −24 m 2  to 0.72×10 −24  m 2 , in the range of 1530 nm to 1540 nm.

DESCRIPTION OF THE INVENTION

[0001] The present application claims priority benefit to U.S. Provisional Application Nos. 60/253,224 and 60/253,225, both of which were filed on Nov. 27, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to optical amplifier fiber modules and optical amplifier fibers.

BACKGROUND OF THE INVENTION

[0003] Optical fibers have attracted a great deal of attention in the telecommunication industry because of their ability to carry larger quantities of information with longer distances between signal boosters than is possible using conventional metal wires. One reason for this is that in conventional wires, such as copper wire and coaxial cable, attenuation increases exponentially with signal frequency, making high-speed transmission over long distances impractical. While light signals in optical fibers also suffer from attenuation as a light signal travels along the fiber, it is significantly less than the loss found in conventional wires. In addition, attenuation in SiO₂ fibers is independent of signal frequency. Both of these advantages help the optical fiber handle more information over larger ranges than is possible in conventional wires.

[0004] In long fiber spans, attenuation may weaken the signal to the point where it cannot excite a photodetector in the receiver. The result is a failed transmission. To cope with this problem, the light signal can be amplified along the length of the fiber span. In optical communication networks, research has focused on two approaches to amplify light signals: repeaters and optical amplifiers.

[0005] A repeater is a device that receives an optical signal, converts the optical signal to an electrical signal, amplifies the electrical signal, and converts the amplified electrical signal back to an optical signal. As will be appreciated, repeaters, are time consuming and lossy. An optical amplifier, on the other hand, amplifies the optical signal without the need for conversion to an electrical signal. Other benefits of optical amplifiers include high gain, low noise, low cross talk and intermodulation distortion, bit-rate transparency and polarization insensitive gain. In the development of optical amplifiers, erbium doped optical amplifier fibers have emerged as the fiber of choice because the characteristic gain bandwidth of these fibers is within a telecommunication window of 1.5 microns (1500 nm), a bandwidth commonly used in fiber optic commercial systems.

[0006] Erbium doped fibers are able to act as optical amplifiers because of their ability to exploit the energy levels of erbium, shown in FIG. 1. For example, when a photon of light, such as a 980 nm pump photon, is directed on a glass doped with Er³⁺, there is a high probability that the pump photon will be absorbed, exciting a ground state ⁴I_(15/2) ion to the ⁴I_(11/2) level. From the ⁴I_(11/2) level, the ion non-radiatively relaxes to the ⁴I_(11/3) level 3, releasing energy as vibrational energy, called phonons. The ⁴I_(13/2) level is metastable, possessing a lifetime of around 10 ms in silica glass. The ion in the metastable ⁴I_(13/2) level eventually emits a photon of light at around 1550 nm during fluorescence, the process whereby the excited electron of the ion radiatively returns to a lower energy level, such as the ground state. A more detailed analysis reveals that in erbium, the ⁴I_(13/2) level actually consists of seven sublevels, and the ⁴I^(15/2) consists of eight sublevels, making 56 possible transitions between the metastable and ground state.

[0007] To illustrate amplification, an Er³⁺ ion in the metastable ⁴I_(13/2) state can be perturbed by a 1550 nm signal photon (before it has had a chance to fluoresce). In this case, the 1550 nm signal photon stimulates the excited ion such that it emits a photon of the same wavelength, in phase, and propagating in the same direction as the stimulating photon. As the Er³⁺ ion returns to the ground state, there will now be two 1550 nm signal photons, the original stimulating photon and the photon emitted from the excited Er³⁺ ion. Amplification is achieved.

[0008] Alternatively, absorption from the ground state to the ⁴I_(13/2) state can also occur. In this case, an incoming 1550 nm signal will be absorbed, exciting some Er³⁺ ions in the ground state. An inversion is created as the Er³⁺ ion population continues to be raised to the excited state. At 100% inversion, no more ions remain in the ground state to absorb incoming photons, and an incoming 1550 nm signal will be strongly amplified.

[0009] As can be seen, gain is limited by the Er³⁺ ion concentration. Problems in fabrication arise when attempting to increase the Er³⁺ ion concentration. One problem is that of clustering, where doped Er³⁺ ions cluster together, destroying an individual atoms ability to generate amplification. Currently, only low Er³⁺ doping concentrations in SiO₂ erbium doped fiber amplifiers (EDFA's) have been achieved in optical amplifier fiber networks, with fiber lengths that exceed tens of meters. One possible solution is discussed in U.S. Pat. No. 4,075,120 to Myers et al. However, Myers only discusses the material composition of glasses with high Er³⁺ doping concentration for making lasers, and not fiber amplification. No phosphate erbium doped glass fiber has been commercialized for EDFA applications. This is primarily due to the inability of researchers to determine correct parameters to develop a successful workable fiber.

[0010] In addition, for amplification, compact and integrated optical amplifiers are desired in the deployment of metro and access optical networks. Known optical amplifiers are designed and assembled based on discrete active and passive optical components including erbium doped optical amplifier fibers, laser diode modules, optical isolators, wavelength division multiplexing couplers, tap couplers, etc. Conventional amplifiers are manufactured using a box-in-a-box approach, where prepackaged devices are coupled together by splicing optical amplifier fibers, also known as fiber pigtails, in order to manufacture optical amplifiers.

[0011] Conventional optical amplifiers may be costly to manufacture and their use in optical networks may result in unwanted optical loss. Additionally, conventional optical amplifiers tend to be rather large, partially due to the fact that the erbium doped optical amplifier fiber in each optical amplifier can be up to tens of meters long. Even if the optical amplifier fiber is coiled up to save space, the bend radius of the fiber still requires a relatively large module package. In order to reduce the size of the optical amplifier module, an integrated solution is needed.

[0012] Thus, there is a need to overcome these and other problems of the related art and to provide an optical amplifier fiber, where the optical amplifier fiber is capable of commercial application. The present invention illustrated in the following description, is directed to solving one or more of the problems set forth above.

SUMMARY OF THE INVENTION

[0013] In accordance with the present invention, a compact optical amplifier module is disclosed which incorporates high gain amplifying gain mediums, such as high gain rare earth doped phosphate glass optical amplifier fibers or waveguides. Components of the optical amplifier module are optically coupled together by free space coupling, as opposed to conventional physical coupling techniques.

[0014] In an exemplary embodiment of the present invention there is an optical fiber amplifier module comprising a signal path located between a signal input and a signal output. A WDM coupler and an amplifying gain medium are optically disposed along the signal path. A pump laser which emits a pump signal is disposed out of the signal path in a manner that allows the pump signal to reflect off the WDM coupler and enter the signal path. The WDM coupler may be placed upstream of the amplifying gain medium, so that the pump signal is reflected into the upstream end of the amplifying gain medium. In an alternative embodiment, the WDM coupler is placed downstream of the amplifying gain medium, so that the pump signal is reflected into the downstream end of the amplifying gain medium. According to a third embodiment, WDM couplers may be placed both upstream and downstream of the amplifying gain medium, so that pump signals may be reflected into both ends of the amplifying gain medium.

[0015] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0016] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a partial energy level diagram for Er³⁺.

[0018]FIG. 2 is a perspective view of an embodiment of an optical fiber amplifier module according to the present invention.

[0019] FIGS. 3A-3C are schematic drawings of exemplary embodiments of optical fiber amplifier modules according to the present invention.

[0020]FIGS. 4A and 4B are graphs showing light amplification vs. length of optical amplifier fiber.

[0021]FIG. 5 is a diagram of an optical amplifier fiber;

[0022]FIG. 6 is an exemplary absorption cross section spectrum for an optical amplifier fiber according to an exemplary embodiment of the present invention;

[0023]FIG. 7 is an exploded diagram of rod and tube method for making an optical amplifier fiber according to an exemplary embodiment of the present invention.

[0024]FIG. 8 is a diagram of the fabrication of an optical amplifier fiber according to an exemplary embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0025] Reference is made to U.S. patent application Ser. No. 09/507,582, filed Feb. 18, 2000, which issued as U.S. Pat. No. 6,292,229 on Sep. 18, 2001; U.S. patent application Ser. No. 09/722,821, filed Nov. 28, 2000; U.S. patent application Ser. No. 09/722,822, filed Nov. 28, 2000; and U.S. Patent Application Serial No. ______ [High Gain Erbium Doped Phosphate Glass Optical Amplifier Fiber, U.S. Provisional Application No. 60/253,225, Assignee reference No.—PHX-0011], which are all owned by the assignee of the present invention and are incorporated by reference herein in their entirety.

[0026] Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0027]FIG. 2 discloses a perspective view of a compact optical amplifier fiber module generally represented by the numeral 10 (hereinafter “module 10”) according to an embodiment of the present invention. Module 10 includes a shell 11 which is generally parallelepiped in shape with a signal input 12 on a first face 14 and a signal output 16, also on first face 14. However, those skilled in the art will recognize that module 10 can have a shape other than parallelepiped, and that signal input 12 and signal output 16 can be on different faces, if desired. In an embodiment of the present invention, a heat sink 20 can be disposed on at least a bottom face 22 of module 10. Heat sink 20 can have a plurality of fins 24 to dissipate heat generated in module 10. In certain embodiments, shell 11 is constructed from a metallized polymer to conduct heat to heat sink 20. However, those skilled in the art will recognize that shell 11 can be formed from other components, such as a non-metallized polymer, or a metal, such as aluminum. Shell 11 can be fabricated from any of several known methods, including, but not limited to, stamping, pressing, or injection molding.

[0028] FIGS. 3A-3C show alternative exemplary embodiments of module 10. In FIG. 3A, a pre-amplified light signal “L” is transmitted along an input fiber 26, which is connected to signal input 12. The light signal L is transmitted through module 10, which increases the energy level of the light signal L, resulting in an amplified light signal L′. The amplified light signal L′ is transmitted to signal output 16, which is connected to an output fiber 28. A signal path 30 is optically disposed within module 10 between signal input 12 and signal output 16. As used herein, the term “signal path” means the path of the light signal L, as amplified to the amplified light signal L′, between input fiber 26 and output fiber 28.

[0029] Module 10 includes both photonic components and electronic components. The photonic components will now be described. Within module 10, a free space optical isolator 40 is disposed along signal path 30, and may be proximate to signal input 12. Optical isolator 40 suppresses backscattered light generated downstream of optical isolator 40 and prevents the backscattered light from traveling upstream to signal input 12. Optical isolators are well known to those skilled in the art and will not be discussed in more detail herein. As used herein, the terms “downstream” and “upstream” are defined as directions from left to right and from right to left, relative to a stated component or components as shown in FIG. 3A, respectively.

[0030] A wavelength division multiplexing (WDM) coupler 50 is disposed within module 10 along signal path 30 downstream of optical isolator 40. WDM coupler 50 allows the light signal L to pass from the upstream side of WDM coupler 50 to the downstream side of WDM coupler 50, but reflects the pump signal P from a pump laser 60, so that the light signal L and the pump signal P are effectively combined together. WDM coupler 50 contemplated in the present embodiment uses a thin film based filter, although those skilled in the art will recognize that other types of WDM couplers can be used.

[0031] In an embodiment of the present invention, pump laser 60 is disposed downstream of WDM coupler 50, but out of signal path 30. Pump laser 60 is not directly coupled to WDM coupler 50, but uses free space coupling to optically couple pump laser 60 to WDM coupler 50. Pump laser 60 is disposed such that a pump signal “P” from pump laser 60 reflects off the downstream side of WDM coupler 50 and enters signal path 30 with the light signal L downstream of WDM coupler 50. The light signal L and the pump signal P are coupled in free space and are collimated for further downstream transmission. In an embodiment, the pump laser can be a single mode laser, but in other embodiments, multimode lasers may also be used. Power for the laser may range, for example, from 20 to 300 milliwatts.

[0032] A high gain amplifier amplifying gain medium 70 is disposed along signal path 30 downstream of WDM coupler 50 and transmits the light signal L from WDM coupler 50 to signal output 16. As the light signal L travels along amplifying gain medium 70, the light signal L is amplified to the light signal L′, which will be described in more detail later herein.

[0033] The amount of amplification is a function of, among other things, the length of the amplifying gain medium, and the power of the pump laser. For example, in an embodiment where amplifying gain medium 70 is 5 cm and where a pump laser having a power of 50 mW is used, the light signal L is amplified approximately 10 dB. A graph showing light signal amplification vs. amplifying gain medium length for a 50 mW pump laser as well as for other, increasing pump powers P₁, P₂, P₃, P₄, P₅ is shown in FIG. 4A. In another exemplary embodiment where amplifying gain medium 70 is 5 cm and where a pump laser having a power of 100 mW is used, the light signal L is amplified approximately 26 dB. A graph showing light signal amplification vs. amplifying gain medium length for a 100 mW pump laser as well as for other, increasing signal pump powers P₁, P₂, P₃ is shown in FIG. 4B. The other parameters for the embodiment of FIG. 4B are shown in TABLE 1 Parameters for a High Gain Erbium Doped Phosphate Glass Amplification Fiber. Parameter Value Erbium doped core diameter 4 μm Numerical aperture of waveguide 0.14 Erbium ion density 1 × 10²¹ cm⁻³ Erbium metastable state lifetime 8 msec Pump wavelength 980 nm Pump direction Co-propagation pump

[0034] The length of the amplifying gain medium may be chosen depending on the amplification desired and the type of amplifying gain medium used. For example, where the amplifying gain medium comprises a doped fiber, the optimum length of the amplifying gain medium may depend on the dopant concentration. In one embodiment, amplifying gain medium 70 can be approximately 2 cm long by using a high gain rare earth doped phosphate glass optical amplifier fiber (REGA). The rare earth concentration may exceed 10²¹ ions/cm³. In other embodiments, amplifying gain medium 70 can be greater or less than 2 cm long, depending on factors such as the material used and the rare earth ion concentration. In certain embodiments, the length of the amplifying gain medium may range from about 1 to 8 cm long.

[0035] Various types of amplifying gain mediums may be used. For example, the amplifying gain medium 70 may comprise an optical amplifier fiber, planar waveguide, or any other light transmitting medium. In certain embodiments, the amplifying gain medium may comprise glass or polymer materials. In further embodiments, the amplifying gain medium may be comprised of dopants, such as, for example, rare earth dopants. In an embodiment of the present invention, amplifying gain medium 70 can be doped with erbium. However, those skilled in the art will recognize that amplifying gain medium 70 can be doped with other elements or combinations of elements, including for example, cerium, praseodymium, lanthanum, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium, and lutetium. Additionally, amplifying gain medium 70 may be manufactured from a gain medium such as an rare earth doped optical polymer, although those skilled in the art will recognize that the amplifying gain medium 70 can be manufactured from other gain media, such as high rare earth concentration glasses.

[0036] Certain embodiments where the amplifying gain medium 70 comprises a waveguide, otherwise known as a planar optical waveguide, will now be described. The waveguide may be either glass or polymeric and may be doped, for example, with rare earth dopants such as erbium. For example, in an embodiment, the waveguide may be a rare earth doped planar waveguide. In a further embodiment, the waveguide may be a vertically integrated waveguide. In certain embodiments, the waveguide may be straight or curved. One example of a curved waveguide may be found in U.S. patent application Ser. No. 09/877,871, filed Jun. 8, 2001 by the present inventor, and entitled “Rare Earth Doped Optical Waveguide And Laser With Optimal Bending Curves”, which is owned by the assignee of the present invention and which is herein incorporated by reference in its entirety. If amplifying gain medium 70 is a waveguide assembly, the output of the waveguide assembly is pigtailed to output fiber 28 in a manner known to those skilled in the art.

[0037] Certain embodiments where the amplifying gain medium 70 comprises an optical amplifier fiber will now be described. The optical amplifier fiber may be either glass or polymeric and may be doped, for example, with rare earth dopants. In other embodiments, the amplifying gain medium 70 can comprise a chirped optical amplifier fiber. For example, in an embodiment, the fiber may comprise a rare earth doped optical polymer.

[0038] Referring to FIG. 5, in an exemplary embodiment the optical amplifier fiber can use a rare earth doped glass. Increasing the rare earth ion concentration can reduce the length of the optical amplifier fiber. In certain embodiments the rare earth doped glass can be a REGA. In an exemplary embodiment, an optical amplifier fiber 505 can be made by fabricating a core and a cladding using a glass melting technique. In this embodiment, a core 510 and a cladding 520 can be fabricated from a rod and tube respectively, for example, when using the rod and tube method, from which the high concentration rare earth doped optical amplifier fiber can be drawn. By selection of materials and fiber parameters of the present invention, amplification can be achieved and tailored to a desired level.

[0039] In certain embodiments of the present invention, rare earth doping of REGA can be accomplished using any of the rare earth elements. For example, the doping can be selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Alternatively, doping can use a combination of two or more of the rare earth elements. It is useful to have the doping in core 510 in the range of 0.5 wt. % to 20 wt. %. In an embodiment the concentration of rare earth dopant in core 510 can be in the range of greater than 3.5 wt. % to 20 wt. %.

[0040] An exemplary embodiment of fabricating the optical amplifier fiber begins by forming a fiber pre-form cladding and a fiber pre-form core. The fiber pre-form core and the fiber pre-form cladding can be made by precision machining of bulk glass materials. In certain embodiments, the bulk glass can be phosphate glass materials. To achieve amplification, core phosphate glass 510 can be doped with the rare earth material. Phosphate prevents clustering of the rare earth dopants, allowing high concentrations of rare earth dopants to be incorporated into the core material. In addition, the inclusion of phosphate allows the rare earth dopants to be uniformly distributed throughout the core. High concentrations and uniform distribution of rare earth elements promotes efficient amplification. In certain embodiments, it is useful to avoid having hydrogen and other Group IA elements in the core. These elements may cause clustering of the rare earth element or quench the radiative lifetimes of the optical amplifier fibers. Similarly, care is taken to prevent O—H or water from being incorporated into the fiber because of their quenching effects.

[0041] Another parameter of the optical amplifier fiber is the diameter ratio, which is the ratio of the diameter of core 510 to the diameter of cladding 520. In an exemplary embodiment, the cladding outer diameter can be in the range of 1 and 5 cm while the diameter of the core can be in the range of 2 μm to 10 μm. In certain embodiments, the core diameter can be in the range of 2 μm to less than 4 μm, or in the range of greater than 6 μm to 10 μm. The diameter ratio can be in the range of 0.04±0.004. The outer diameter of the fiber can be in the range of 100 μm to 150 μm.

[0042] Adjusting the refractive indices of core 510 and cladding 520 allows the numerical aperture (NA) of the fiber 505 to be adjusted. NA is defined by the following formula:

NA=(n _(core) ² −n _(cladding) ²)^(½)  (1)

[0043] where n_(core) is the core index of refraction and n_(cladding) is the cladding index of refraction. Correspondingly, NA can be adjusted by adjusting either or both of N_(core) or N_(cladding).

[0044] In an exemplary embodiment, when the NA is in the range of 0.1 to 0.3, single mode propagation of the pump and signal can be achieved. In certain embodiments, NA can be in the range of 0.04 to less than 0.216, and in other embodiments, NA can be in the range of greater than 0.216 to 0.3.

[0045] Different dopants can be used to adjust the index of refraction of the material. In an exemplary embodiment, germanium can be used to adjust the index of refraction of either or both of the core and the cladding. In other embodiments, the doping can be aluminum, or combinations of germanium and aluminum. Similarly, lanthanum has been found to aid in fabricating optical amplifier fibers and it can be used separately, or in combination with aluminum and/or germanium.

[0046] In an embodiment of the present invention, when the materials and fiber parameters are selected, the spectroscopic and thermal properties of the optical amplifier fiber can be tailored. For example, according to an exemplary embodiment of the present invention, the radiative lifetime of the core can be in the range of 7.0 to 9.0 milliseconds at 1535 nm. In addition, the fluorescence lifetime of the core can be greater than 7.5 milliseconds at 1535 nm.

[0047] Thermal properties that can be controlled include transformation point and the coefficient of thermal expansion. In an embodiment of the present invention, the transformation point difference between core 510 and cladding 520, measured in (°C.), can be less than 5%. The thermal expansion coefficient difference between core 510 and cladding 520, measured in (/°C), can be less than 2%.

[0048] In an exemplary embodiment of the present invention where the materials and fiber parameters are adjusted, the absorption cross section can be controlled. The absorption cross section can be tailored to be in the range of 0.60×10⁻²⁴ m² to 0.72×10⁻²⁴ m² , in the range of 1530 nm to 1540 nm. In certain embodiments, the absorption cross section can be in the range of about 0.61×10⁻²⁴ m² to 0.65×10⁻²⁴ m², in the range of about 1530 nm to 1540 nm, or in the range of about 0.62×10⁻²⁴ m² to 0.64×10⁻²⁴ m², in the range of about 1530 nm 1540 nm.

[0049]FIG. 6 shows a graph of the measured absorption cross section of a fiber made according to the embodiments of the present invention. In this embodiment, the absorption cross section is approximately 0.625×10⁻²⁴ m² at 1535 nm.

[0050] In an embodiment of the present invention, materials and fiber parameters to fabricate the REGA can be chosen using a computer program. The computer program can consider the desired gain and design a set of manufacturing steps to accomplish the fiber. In an embodiment, there can be a computer readable medium containing instructions for controlling a computer-aided manufacturing system to perform a method for manufacturing optical amplifier fibers and waveguides of the present invention.

[0051] Shown in FIGS. 7 and 8 are exemplary methods of making the optical amplifier fiber. In FIG. 7, the optical amplifier fiber is made by the rod and tube method. In this embodiment, a phosphate glass rare earth doped rod 710 and a phosphate glass tube 720 are formed. Different methods can be used to make rod 710 and tube 720, such as casting molten glass into molds, machining a bulk starting material block, or rotational casting.

[0052] In an embodiment, rod 710 is doped with the rare earth element while tube 720 remains substantially free from rare earth doping. In addition, rod 710 and tube 720 can doped with the index doping material at the levels needed to achieve the desired NA. As can be seen from FIG. 7, rod 710 is inserted into tube 720. The resulting rod and tube structure can be thermally annealed to form a pre-form 830, shown in FIG. 8.

[0053] In FIG. 8, a fiber 840 is drawn from preform 830 and fed through a furnace 850. Furnace 850 heats drawn fiber 840 to a temperature between its glass transition temperature and its melting point. Passing through furnace 850 softens rod 710 and tube 720. The softening smoothes roughness on the outside surface of rod 710 and the inside surface of tube 720 and aids in bonding the two together. Rod 710 and tube 720 become core 510 and cladding 520, respectively, of the optical amplifier fiber 505, shown in FIG. 5. Fiber 840 is directed through a diameter monitor 860 to accurately control the diameter of optical amplifier fiber 505. In an exemplary embodiment, fiber 840 passes through a coater 870 to deposit a protective coating over fiber 840. In certain embodiments, the coating can be a polymer. Once fiber 840 has been coated, it can be wound on a winding drum 880 for safe storage and easy dispensing.

[0054] Referring back to FIG. 3A, a plurality of lenses 80 are disposed within module 10 along signal path 30 between signal input 12 and signal output 16. Lenses 80 collimate and couple the light signal L and the pump signal P. Lenses 80 can be spherical, aspherical, or graded index. As shown in FIG. 3A, a first lens 82 is optically disposed between signal input 12 and optical isolator 40. A second lens 84 is optically disposed between optical isolator 40 and WDM coupler 50. Third and fourth lenses 86, 88 are disposed between the WDM coupler 50 and the amplifying gain medium 70. Although the locations of the lenses 80 are as described, those skilled in the art will recognize that some of the lenses 80 can be omitted and/or additional lenses can also be disposed along signal path 30 throughout module 10.

[0055] In an embodiment of the present invention, the components such as signal input 12, optical isolator 40, WDM coupler 50, pump laser 60, amplifying gain medium 70 and lenses 80 are optically coupled by free space coupling, although those skilled in the art will recognize that some or all of the components can be physically coupled as well. In an embodiment, amplifying gain medium 70 is physically coupled to signal output 16 and signal output 16 is physically coupled to output fiber 28. However, those skilled in the art will recognize that amplifying gain medium 70, signal output 16, and output fiber 28 can be coupled together by free space coupling as well.

[0056] The electronic components of module 10 will now be described. A terminal port 90 is disposed on a face other than first face 14. However, those skilled in the art will recognize that terminal port 90 can be disposed on first face 14. In an embodiment, terminal port 90 is an RS-232 connector, which is well known in the art. Terminal port 90 is electronically connected to pump laser 60 through a control board 65 in module 10, so that electronic and power signals generated by a controller/power supply (not shown) outside module 10 can control operation of pump laser 60 through terminal port 90 and control board 65. Control board 65 is disposed proximate heat sink 20 so that heat sink 20 can dissipate heat generated by control board 65 during operation of module 10.

[0057] In an embodiment of the present invention, module 10 is approximately 6 cm×1 cm×1 cm in size, although those skilled in the art will recognize that module 10 can be of different dimensions. In certain embodiments, however, the overall size of module 10 can be less than 10 cm³.

[0058] Referring to FIG. 3A, in operation, the pre-amplified light signal L is provided along input fiber 26 to signal input 12. The light signal L has a predetermined bandwidth, such as a 0.1 nm range between 1520 nm and 1560 nm, although those skilled in the art will recognize that light signal L can have a larger bandwidth and have other wavelengths as well. The light signal L travels along signal path 30 to first lens 82, which collimates the light signal L and directs the light signal L along signal path 30 toward optical isolator 40. The light signal L travels through optical isolator 40 and along signal path 30 to second lens 84. Second lens 84 again collimates the light signal L and directs the light signal L along signal path 30 to WDM coupler 50. The light signal L passes through WDM coupler 50 along signal path 30 toward third lens 86.

[0059] Simultaneously to the light signal L passing through WDM coupler 50, pump laser 60 emits the pump signal P, which is aimed at the downstream side of WDM coupler 50. In an embodiment, the pump signal P has a wavelength of either approximately 980 nm or 1480 nm, although those skilled in the art will recognize that other wavelengths can be used. The pump signal P reflects off WDM coupler 50 and enters signal path 30, where the pump signal P is aligned with the light signal L along the signal path 30. Pump laser 60 is controlled by control board 65, which is connected to the controller (not shown) through terminal port 90. Control board 65 controls the intensity of the pump signal P.

[0060] The combined light signal L and pump signal P travel along optical path 30 to and through third and fourth lenses 86, 88, which collimate the combined light and pump signals L, P and direct the combined light and pump signals L, P along signal path 30 to amplifying gain medium 70. Light from the pump signal P excites dopant ions in amplifying gain medium 70, raising them to a higher energy level as shown in FIG. 1.

[0061] With reference to FIG. 1, in the amplifying gain medium 70, the dopant ions start out in their ground state, level 1. The electrons in the ions are then excited to level 2 by the pump beam P, equal to the transition energy from 1→2. The ions subsequently undergo fast nonradiative decay to level 3, which is the metastable state of the system. The lifetime of this state is very long in comparison to the nonradiative decay. As a consequence, a population inversion is created in level 3. Then, as the light signal L passes by the ions, the light signal L stimulates emission of photons from the ions with the same signal energy. This stimulated decay is from level 3 to level 1, the ground state. The excited ions emit the photons at the same wavelength as the light signal L and in phase with the light signal L, thus amplifying the light signal L to the amplified light signal L′. The light signal L′ continues to increase in strength along the amplifying gain medium 70 while the pump signal P is depleted.

[0062] The amplified light signal L′ then exits amplifying gain medium 70 to signal output 16, where the amplified light signal L′ continues along output fiber 28 to another component, such as another amplifier or to an end component, such as a de-multiplexer.

[0063] Alternative embodiments to the embodiment just described for FIG. 3A are illustrated in FIGS. 3B and 3C. In the embodiment of FIG. 3B, WDM coupler 50 is disposed downstream of amplifying gain medium 70. In this embodiment, pump laser 60 is disposed so that the pump signal “P” is reflected off the upstream side of the WDM coupler 50 and enters the amplifying gain medium 70 at the downstream end. The WDM coupler 50 is selected to allow the amplified light signal L′ to pass through the WDM coupler 50. In the embodiment of FIG. 3C, two WDM couplers are disposed along the signal path. One WDM coupler is disposed upstream of the amplifying gain medium and the other is disposed downstream of the amplifying gain medium. In this manner the pump signal P1 emitted from a first pump laser 60 may enter the amplifying gain medium at the upstream end, while the second pump signal P2 from a second pump laser 60 may enter the amplifying gain medium at the downstream end.

[0064] Those skilled in the art will recognize that module 10 can be fabricated to be used in a variety of environments, such as in long-haul terrestrial, submarine, access, metro, local area network (LAN), and other types of applications. For a module 10 using a high gain erbium doped fiber, a −10 dB optical signal can be transmitted up to approximately 50 km between amplifier modules 10.

[0065] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. An optical amplifier fiber comprising: a core manufactured from a phosphate glass doped with a rare earth element: a cladding manufactured from a phosphate glass surrounding the core; wherein an absorption cross section of the optical amplifier fiber is in the range of 0.60×10⁻²⁴ m² to 0.72×10⁻²⁴ m², in the range of 1530 nm to 1540 nm.
 2. An optical amplifier fiber according to claim 1 wherein the rare earth element is selected from at least one of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
 3. An optical amplifier fiber of claim 1 wherein the rare earth element a combination of erbium and ytterbium.
 4. An optical amplifier fiber of claim 1 wherein the absorption cross section of the optical amplifier fiber is in the range of about 0.61×10⁻²⁴ m to 0.65×10⁻²⁴ m, in the range of about 1530 nm to 1540 nm.
 5. An optical amplifier fiber of claim 1 wherein the absorption cross section of the optical amplifier fiber is in the range of about 0.62×10⁻²⁴ m² to 0.64×10⁻²⁴ m², in the range of about 1530 nm to 1540 nm.
 6. An optical amplifier fiber of claim 1 wherein a numerical aperture of the optical amplifier fiber is in the range of 0.1 to 0.3.
 7. An optical amplifier fiber of claim 1 wherein a numerical aperture of the optical amplifier fiber is in the range of 0.04 to less than 0.216.
 8. An optical amplifier fiber of claim 1 wherein a numerical aperture of the optical amplifier fiber is in the range of greater than 0.216 to 0.3.
 9. An optical amplifier fiber of claim 1 wherein a concentration of rare earth elements is in the range of greater than 3.5 wt. % to 20 wt. %.
 10. An optical amplifier fiber of claim 1 wherein a diameter of the core is in the range of 2 μm to 10 μm.
 11. An optical amplifier fiber of claim 1 wherein a diameter of the core is in the range of 2 μm to less than 4 μm.
 12. An optical amplifier fiber of claim 1 wherein a diameter of the core is in the range of greater than 6 μm to 10 μm.
 13. An optical amplifier fiber according to claim 1 wherein at least one of the core and the cladding includes an index adjusting doping material.
 14. An optical amplifier fiber according to claim 13 wherein the index adjusting doping material is germanium.
 15. An optical amplifier fiber according to claim 14 wherein the index adjusting doping material is selected from at least one of germanium, aluminum, and lanthanum.
 16. An optical amplifier fiber according to claim 1, wherein the core has a radiative lifetime in the range of 7 to 9 milliseconds at 1535 nm.
 17. An optical amplifier fiber according to claim 1, wherein the core has a fluorescence lifetime of greater than 7.5 milliseconds at 1535 nm.
 18. An optical amplifier fiber according to claim 1, wherein the optical amplifier fiber has a diameter ratio in the range of 0.036 to 0.044.
 19. An optical amplifier fiber according to claim 1, wherein a transformation point difference of the core and the cladding, measured in (°C.), is less than 5%.
 20. A method of making an optical amplifier fiber comprising: providing a pre-form having a rare earth doped core, and a cladding; drawing down the pre-form to form an optical amplifier fiber; wherein an absorption cross section of the optical amplifier fiber is in the range of 0.60×10⁻²⁴ m² to 0.72×10−24 m², in the range of 1530 nm to 1540 nm.
 21. A method of making an optical amplifier fiber according to claim 20, wherein the rare earth doped core has a radiative lifetime in the range of 7 milliseconds to 9 milliseconds.
 22. A method of making an optical amplifier fiber according to claim 20, wherein the core has a fluorescence lifetime of at least 7.5 milliseconds.
 23. A method of making an optical amplifier fiber according to claim 20 wherein the optical amplifier fiber has a transformation point difference between the core and the cladding, measured in (°C.), is less than 5%.
 24. A method of making an optical amplifier fiber according to claim 20 wherein the optical amplifier fiber is made by a rod and tube method.
 25. A method of making an optical amplifier fiber according to claim 20 wherein the optical amplifier fiber is made by a rotational casting method.
 26. A method of making an optical amplifier fiber according to claim 20, wherein the rare earth element is selected from at least one of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
 27. A method of making an optical amplifier fiber according to claim 20 wherein the absorption cross section of the optical amplifier fiber is in the range of about 0.61−10⁻²⁴ m² to 0.65×10⁻²⁴ m², in the range of about 1530 nm to 1540 nm.
 28. A method of making an optical amplifier fiber according to claim 20 wherein the absorption cross section of the optical amplifier fiber is in the range of about 0.62×10⁻²⁴ m² to 0.64×10⁻²⁴ m², in the range of about 1530 nm to 1540 nm.
 29. A method of making an optical amplifier fiber according to claim 20 wherein a numerical aperture of the optical amplifier fiber is in the range of 0.1 to 0.3.
 30. A method of making an optical amplifier fiber according to claim 20 wherein a numerical aperture of the optical amplifier fiber is in the range of 0.04 to less than 0.216.
 31. A method of making an optical amplifier fiber according to claim 20 wherein a numerical aperture of the optical amplifier fiber is in the range of greater than 0.216 to 0.3.
 32. A method of making an optical amplifier fiber according to claim 20 wherein a concentration of rare earth elements is in the range of greater than 3.5 wt. % to 20 wt. %.
 33. A method of making an optical amplifier fiber according to claim 20 wherein a diameter of the core is in the range of 2 μm to 10 μm.
 34. A method of making an optical amplifier fiber according to claim 20 wherein a diameter of the core is in the range of 2 μm to less than 4 μm.
 35. A method of making an optical amplifier fiber according to claim 20 wherein a diameter of the core is in the range of greater than 6 μm to 10 μm.
 36. A method of making an optical amplifier fiber according to claim 20 wherein at least one of the core and the cladding include an index adjusting doping material.
 37. An optical amplifier fiber according to claim 36 wherein the index adjusting doping material is germanium.
 38. An optical amplifier fiber according to claim 36 wherein the index adjusting doping material is selected from at least one of germanium, aluminum, and lanthanum.
 39. A method of amplifying an optical signal in an optical amplifier comprising: providing an optical amplifier including an optical amplifier fiber having a core manufactured from phosphate glass doped with a rare earth element selected from at least one of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; wherein an absorption cross section of the optical amplifier fiber is in the range of 0.60×10⁻²⁴ m² to 0.72×10⁻²⁴ m², in the range of 1530 nm to 1540 nm; surrounding the core with a cladding, the cladding manufactured from phosphate glass, wherein the cladding is substantially free of the rare earth element; providing a pre-amplified signal to the optical amplifier; and amplifying the pre-amplified signal in the optical amplifier.
 40. A method of amplifying an optical signal in an optical amplifier according to claim 39, wherein a concentration of the at least one rare earth element is in the range of 0.5 wt. % to 20 wt. %.
 41. A method of amplifying an optical signal in an optical amplifier according to claim 39, wherein the rare earth doped core has a radiative lifetime in the range of 7 milliseconds to 9 milliseconds;
 42. A method of amplifying an optical signal in an optical amplifier according to claim 39, wherein the core has a fluorescence lifetime of at least 7.5 milliseconds;
 43. A method of amplifying an optical signal in an optical amplifier according to claim 39, wherein the optical amplifier fiber has a transformation point difference between the core and the cladding, measured in (°C.), is less than 5%; and
 44. A method of amplifying an optical signal in an optical amplifier according to claim 39, further comprising: providing a pump signal to the optical amplifier fiber.
 45. A method of amplifying an optical signal in an optical amplifier according to claim 39, wherein amplifying the pre-amplified signal comprises transmitting the pre-amplified signal through the optical amplifier fiber over an amplification length of less than 10 centimeters.
 46. A computer readable medium containing instructions for a method for manufacturing an optical amplifier fiber, the method comprising: receiving a pre-form having a rare earth doped core, and a cladding; drawing down the pre-form to form an optical amplifier fiber; wherein an absorption cross section of the optical amplifier fiber is in the range of 0.60×10−24 m² to 0.72×10⁻²⁴ m², in the range of 1530 nm to 1540 nm.
 47. A computer readable medium according to claim 46, wherein the rare earth doped core has a radiative lifetime in the range of 7 milliseconds to 9 milliseconds.
 48. A computer readable medium according to claim 46, wherein the core has a fluorescence lifetime of at least 7.5 milliseconds.
 49. A computer readable medium according to claim 46, wherein the optical amplifier fiber has a transformation point difference between the core and the cladding, measured in (°C.), is less than 5%. 